5 ms·

Just wait until we reach the proper space age and start traveling at speeds close to the speed of light. Taking into account the relativistic effects is going m

Just wait until we reach the proper space age and start traveling at speeds close to the speed of light. Taking into account the relativistic effects is going make more than a few coders switch carees to organic farming.

nradov 5y ago
GPS satellite designers already have to account for clock skew due to relativistic effects. A nice little real world confirmation of the theory.
monocasa 5y ago
Luckily clock skew already happens and is handled more or less by the current systems. The difference between the system moving at relativistic speeds versus a crystal being slightly out of spec add up to the same result. Incorrect number of ticks for a given proper time interval. That's part of the genius of Leslie Lamport, embracing the fact that your distributed clocks aren't consistent, so start looking at effects through distributed systems from a light cone perspective just like you do with relativity.
- willis936 5y ago
  Clock skew is in error. Relativistic time dilation is not. Both clocks will be right and disagree. You can't escape answering the question: do I care about the time elapsed at home or the time elapsed here?
  - kuschku 5y ago
    We already define time relative to 1970-01-01, relative to the mean solar noon in Greenwich. We should also define the length of a second to mean "at sea level in Greenwich".
    - tialaramex 5y ago
      This just means the "length of a second" wanders around randomly and is thus useless. So now "a second" isn't any particular amount of time, and you've lost all ability to really measure time except in a vague inexact sense. The quest to eliminate irreproducibility from metrication is why SI did all that work to finally eliminate the kilogram prototype. The second was defined reproducibly decades ago because trying to define it based on our planet's inconstant spin was such a failure. The fact that the Earth's spin is not constant is perhaps frustrating, but like other facts just insisting it isn't true doesn't help you.
      - kuschku 5y ago
        > This just means the "length of a second" wanders around randomly and is thus useless Why would it? My redefinition just means the length of a second is defined as a given number of cycles of caesium in a gravitational field equivalent to that of Greenwich. If you're in a different gravitational field, you'll just have to convert the values easily.
      - fanf2 5y ago
        Right, but general relativity matters for how astronomers define their timescales, and how they are measured and maintained in practice: there are different scales for barycentric time (at the centre of gravity of the solar system), geocentric time (centre of the earth) and terrestrial time (on the surface of the rotating geoid). TAI is supposed to tick at the same rate as TT, so it depends on a model of an equal-gravitation version of sea level (the geoid) and the relative height of the time labs. In particular, NIST is a mile high in Colorado so their clocks tick a lot faster than (say) NPL by the Thames in west London.
        tialaramex 5y ago
        Sure, the quest for Universal Time is despite the name necessarily a local quest so you can't meaningfully export your results to creatures orbiting a distant quasar as you can with the SI units. But the size of the discrepancy between Colorado and London is far smaller than the discrepancy Leap Seconds are concerned with right?
    - kempbellt 5y ago
      Sea level when the tide is in, or out?
      - fanf2 5y ago
        “sea level” is short for “mean sea level”
        kempbellt 5y ago
        I mean, I feel like that's fairly obvious, and the playful pedantry of my comment in a discussion on the very definition of basic unit of measurement was overlooked. If we are going so far as to rigidly include "sea level" in the definition of "a second", it would probably help if we say that that we mean mean sea level. Know what I mean?
        el_isma 5y ago
        Not trying to troll, but... What happens if/when global warming makes sea level rise? Do we have measurements that are still based on "sea level"?
        _whiteCaps_ 5y ago
        https://tidesandcurrents.noaa.gov/sltrends/faq.html https://tidesandcurrents.noaa.gov/sltrends/faq.html > Tidal datums must be updated at least every 20-25 years due to global sea level rise. Some stations are more frequently updated due to high relative sea level trends.
    - CamperBob2 5y ago
      These days, the length of a second is defined by the difference in frequency between two ground-state energy levels of a caesium-133 atom, which works out to 9192631770 cycles per second. No need to check tide tables.
      - kuschku 5y ago
        That definition is still relative. It still requires as additional factor the place where the measurement is taken, due to Einstein's theories. That's why I proposed changing the second definition to 9192631770 cycles in a gravitational field equivalent to that of Greenwich.
  - dasil003 5y ago
    If you need time elapsed then you implicitly need a frame of reference in the same way that a clock time needs a time zone to be meaningful in a global basis. In practice if you wanted this it would mean having a timer colocated within your frame of reference as there is no scalar representation of a frame of reference that would allow you to do timestamp calculations. Timestamp math only works as a hack because we mostly stay relativistically close to Greenwich.
    - fanf2 5y ago
      Astronomers use mathematical models of the solar system to translate between barycentric time, geocentric time, and terrestrial time, which are all different relativistic frames of reference. Only TT has actual clocks: TAI ticks at the same rate as TT, tho for historical reasons there is a fixed offset. That isn’t counting the clocks derided from observing the orbits of the moons and planets…
  - monocasa 5y ago
    At the end of the day there's always skew; crystals in systems aren't made to perfect tolerances. In fact I'm not even sure how you would define perfect tolerances for clocks in a meaningful way. So at the end of the day it doesn't matter if it's in 'error' or a fundamental to the physics even under perfect conditions. Neither are really under your control and both have the same answer. > You can't escape answering the question: do I care about the time elapsed at home or the time elapsed here? We already have that concept, in the difference between CLOCK_WALL and CLOCK_MONOTONIC. You use monotonic for "local temporal reference frame" and wall for "proxy of globally visible temporal reference frame". Wall shifts around anyway as NTP or what have you figures out how skewed off of "actual" time you are.
  - Dylan16807 5y ago
    > You can't escape answering the question Yes you can. When error swamps the difference between two things, you don't have to choose. Do you set your analog alarm clock to wake you at 6:00:00 or 6:00:05?
    - willis936 5y ago
      Is that not choosing?
      - Dylan16807 5y ago
        The answer I was implying is: You can't distinguish between them, you put the arm in the vague vicinity of both. So no it's not choosing. You could choose which to aim at if you really wanted to, but the end result would be basically the same. You can't choose which one it's set to, because it's so far below the noise floor.
        willis936 5y ago
        Today it is. My point is if a ship goes on an 80 year journey at an appreciable speed then its wall clock will disagree by an appreciable amount. If we have hundreds or thousands of such ships and they communicate with each other, someone will need to choose what clock they care about and it won't be the Earth's clock.
        Dylan16807 5y ago
        Well I meant the real you/me. Not a hypothetical spaceman. Anyone on Earth can just say their computer clocks are set to "Earth", ignore relativity, and use their normal clock skew correction algorithms. It's not "You can't escape answering the question". It's "Interplanetary spacemen can't escape answering the question." And those spacemen might not even use seconds.
gorjusborg 5y ago
I'd like to think my descendants will be beet farmers in a far away solar system.
ISL 5y ago
Those corrections (both special and general relativistic, with opposite signs) are already required to allow GPS to function.
eftychis 5y ago
We do: all these are taken into account for satellites. In particular GPS is one of the prime examples as it is affected by both special and general relativity and how fast time elapses is critical to its function.
Swenrekcah 5y ago
Probably takes some serious skills to be an organic farmer on Ceres.
- bee_rider 5y ago
  Conveniently it turns out that the results are the same independent of your skill.
- labster 5y ago
  It does, but you make more money than you would using standard farming methods and pesticides on Ceres.

nradov 5y ago

GPS satellite designers already have to account for clock skew due to relativistic effects. A nice little real world confirmation of the theory.

monocasa 5y ago

Luckily clock skew already happens and is handled more or less by the current systems. The difference between the system moving at relativistic speeds versus a crystal being slightly out of spec add up to the same result. Incorrect number of ticks for a given proper time interval. That's part of the genius of Leslie Lamport, embracing the fact that your distributed clocks aren't consistent, so start looking at effects through distributed systems from a light cone perspective just like you do with relativity.

willis936 5y ago
Clock skew is in error. Relativistic time dilation is not. Both clocks will be right and disagree. You can't escape answering the question: do I care about the time elapsed at home or the time elapsed here?
- kuschku 5y ago
  We already define time relative to 1970-01-01, relative to the mean solar noon in Greenwich. We should also define the length of a second to mean "at sea level in Greenwich".
  - tialaramex 5y ago
    This just means the "length of a second" wanders around randomly and is thus useless. So now "a second" isn't any particular amount of time, and you've lost all ability to really measure time except in a vague inexact sense. The quest to eliminate irreproducibility from metrication is why SI did all that work to finally eliminate the kilogram prototype. The second was defined reproducibly decades ago because trying to define it based on our planet's inconstant spin was such a failure. The fact that the Earth's spin is not constant is perhaps frustrating, but like other facts just insisting it isn't true doesn't help you.
    - kuschku 5y ago
      > This just means the "length of a second" wanders around randomly and is thus useless Why would it? My redefinition just means the length of a second is defined as a given number of cycles of caesium in a gravitational field equivalent to that of Greenwich. If you're in a different gravitational field, you'll just have to convert the values easily.
    - fanf2 5y ago
      Right, but general relativity matters for how astronomers define their timescales, and how they are measured and maintained in practice: there are different scales for barycentric time (at the centre of gravity of the solar system), geocentric time (centre of the earth) and terrestrial time (on the surface of the rotating geoid). TAI is supposed to tick at the same rate as TT, so it depends on a model of an equal-gravitation version of sea level (the geoid) and the relative height of the time labs. In particular, NIST is a mile high in Colorado so their clocks tick a lot faster than (say) NPL by the Thames in west London.
      - tialaramex 5y ago
        Sure, the quest for Universal Time is despite the name necessarily a local quest so you can't meaningfully export your results to creatures orbiting a distant quasar as you can with the SI units. But the size of the discrepancy between Colorado and London is far smaller than the discrepancy Leap Seconds are concerned with right?
  - kempbellt 5y ago
    Sea level when the tide is in, or out?
    - fanf2 5y ago
      “sea level” is short for “mean sea level”
      - kempbellt 5y ago
        I mean, I feel like that's fairly obvious, and the playful pedantry of my comment in a discussion on the very definition of basic unit of measurement was overlooked. If we are going so far as to rigidly include "sea level" in the definition of "a second", it would probably help if we say that that we mean mean sea level. Know what I mean?
      - el_isma 5y ago
        Not trying to troll, but... What happens if/when global warming makes sea level rise? Do we have measurements that are still based on "sea level"?
        _whiteCaps_ 5y ago
        https://tidesandcurrents.noaa.gov/sltrends/faq.html https://tidesandcurrents.noaa.gov/sltrends/faq.html > Tidal datums must be updated at least every 20-25 years due to global sea level rise. Some stations are more frequently updated due to high relative sea level trends.
  - CamperBob2 5y ago
    These days, the length of a second is defined by the difference in frequency between two ground-state energy levels of a caesium-133 atom, which works out to 9192631770 cycles per second. No need to check tide tables.
    - kuschku 5y ago
      That definition is still relative. It still requires as additional factor the place where the measurement is taken, due to Einstein's theories. That's why I proposed changing the second definition to 9192631770 cycles in a gravitational field equivalent to that of Greenwich.
- dasil003 5y ago
  If you need time elapsed then you implicitly need a frame of reference in the same way that a clock time needs a time zone to be meaningful in a global basis. In practice if you wanted this it would mean having a timer colocated within your frame of reference as there is no scalar representation of a frame of reference that would allow you to do timestamp calculations. Timestamp math only works as a hack because we mostly stay relativistically close to Greenwich.
  - fanf2 5y ago
    Astronomers use mathematical models of the solar system to translate between barycentric time, geocentric time, and terrestrial time, which are all different relativistic frames of reference. Only TT has actual clocks: TAI ticks at the same rate as TT, tho for historical reasons there is a fixed offset. That isn’t counting the clocks derided from observing the orbits of the moons and planets…
- monocasa 5y ago
  At the end of the day there's always skew; crystals in systems aren't made to perfect tolerances. In fact I'm not even sure how you would define perfect tolerances for clocks in a meaningful way. So at the end of the day it doesn't matter if it's in 'error' or a fundamental to the physics even under perfect conditions. Neither are really under your control and both have the same answer. > You can't escape answering the question: do I care about the time elapsed at home or the time elapsed here? We already have that concept, in the difference between CLOCK_WALL and CLOCK_MONOTONIC. You use monotonic for "local temporal reference frame" and wall for "proxy of globally visible temporal reference frame". Wall shifts around anyway as NTP or what have you figures out how skewed off of "actual" time you are.
- Dylan16807 5y ago
  > You can't escape answering the question Yes you can. When error swamps the difference between two things, you don't have to choose. Do you set your analog alarm clock to wake you at 6:00:00 or 6:00:05?
  - willis936 5y ago
    Is that not choosing?
    - Dylan16807 5y ago
      The answer I was implying is: You can't distinguish between them, you put the arm in the vague vicinity of both. So no it's not choosing. You could choose which to aim at if you really wanted to, but the end result would be basically the same. You can't choose which one it's set to, because it's so far below the noise floor.
      - willis936 5y ago
        Today it is. My point is if a ship goes on an 80 year journey at an appreciable speed then its wall clock will disagree by an appreciable amount. If we have hundreds or thousands of such ships and they communicate with each other, someone will need to choose what clock they care about and it won't be the Earth's clock.
        Dylan16807 5y ago
        Well I meant the real you/me. Not a hypothetical spaceman. Anyone on Earth can just say their computer clocks are set to "Earth", ignore relativity, and use their normal clock skew correction algorithms. It's not "You can't escape answering the question". It's "Interplanetary spacemen can't escape answering the question." And those spacemen might not even use seconds.

gorjusborg 5y ago

I'd like to think my descendants will be beet farmers in a far away solar system.

ISL 5y ago

Those corrections (both special and general relativistic, with opposite signs) are already required to allow GPS to function.

eftychis 5y ago

We do: all these are taken into account for satellites. In particular GPS is one of the prime examples as it is affected by both special and general relativity and how fast time elapses is critical to its function.

Swenrekcah 5y ago

Probably takes some serious skills to be an organic farmer on Ceres.

bee_rider 5y ago
Conveniently it turns out that the results are the same independent of your skill.
labster 5y ago
It does, but you make more money than you would using standard farming methods and pesticides on Ceres.